As typically seen in a liquid crystal display device, a transistor formed over a glass substrate or the like is formed using amorphous silicon, polycrystalline silicon, or the like. Although transistors including amorphous silicon have low field-effect mobility, they can be formed over a larger glass substrate. In contrast, transistors including polycrystalline silicon have high field-effect mobility, but they have a disadvantage of not being suitable for a large glass substrate.
In contrast to the transistors manufactured using silicon, attention has been drawn to a technique by which a transistor is manufactured using an oxide semiconductor and is applied to an electronic device or an optical device. For example, Patent Document 1 and Patent Document 2 disclose a technique by which a transistor is manufactured using zinc oxide or In—Ga—Zn—O-based oxide as an oxide semiconductor and is used as a switching element of pixel or the like of an image display device.